Ten yearling Boer goat wethers (45.4 ± 0.92 kg) consuming fresh Sudangrass ad libitum while grazing (GRA) a 0.8-ha pasture or individually confined (CON) were used in a crossover experiment with 3-wk periods to evaluate factors influencing estimates of energy used for activity (AEC) when grazing. Fresh forage offered to CON wethers was 15.9 and 13.4% CP and 65.0 and 67.4% NDF in periods 1 and 2, respectively. Based on forage and fecal AIA, forage DE concentration for CON averaged 67.9 and 56.5% in periods 1 and 2, respectively. From these values and fecal DM, least squares means of ME intake were 405 and 484 kJ/kg BW0.75 for CON and GRA, respectively (SE = 15.4). Heat energy (HE) determined from heart rate (HR) measured over 1 d and the ratio of HE to HR estimated earlier was less (P < 0.001) for CON than for GRA (482 and 642 kJ/kg BW0.75; SE = 17.2). The AEC estimated by subtraction from HE of ME used for maintenance (MEm; 427 kJ/kg BW0.75), HE expended for tissue energy gain based on recovered energy (RE) when greater than 0, an efficiency of ME use for gain (i.e., [0.0423 × forage ME in MJ/kg DM] + 0.006; 0.40 ± 0.009), and the same efficiency of use for maintenance (km; [0.019 × forage ME in MJ/kg DM] + 0.503; 0.68 ± 0.004) of energy from forage and mobilized tissue with RE less than 0 was 39 and 213 kJ/kg BW0.75 for CON and GRA, respectively (SE = 21.9). Slightly greater values resulted from assuming that mobilized tissue energy was used for maintenance more efficiently (i.e., 0.80) than forage ME, with values of 57 and 241 kJ/kg BW0.75 for CON and GRA, respectively (SE = 23.9). The former AEC value for GRA and that determined simply from the difference between GRA and CON HE were much greater than AEC based on time spent in different activities (i.e., resting-lying, resting-standing, grazing, and walking) multiplied by corresponding HE and assuming that AEC resulted from HE when resting-standing, grazing, and walking (217 ± 19.7, 165 ± 19.3, and 46 ± 4.85 kJ/kg BW0.75, respectively). In conclusion, determining the AEC of meat goats while grazing by subtraction of other sources of HE is influenced by specific assumptions of energy requirements and efficiencies of use for different physiological functions.